1. Field of the Invention
The present invention relates generally to the field of x-ray optics and x-ray imaging. More specifically, the present invention discloses an x-ray interferometer for use in telescopes, microscopes and spectrometers.
2. Statement of the Problem
The creation of very high resolution x-ray optics is a formidable technical challenge, but one that holds promise of great scientific reward to the scientist. Such a device allows the astronomer to image x-ray sources in the sky with ultra-high resolution, the microscopist to study finer features, and both to perform sensitive spectroscopy of targets.
Even with optics of arbitrarily good performance, the effects of diffraction limit resolution. One way to achieve higher resolution is to exploit an approach that is widely used in the radio and optical spectral regions--interferometry. We show that a broadband x-ray interferometry is possible using current technology, and we have established the viability of this approach using laboratory measurements of a prototype interferometer.
For astronomers, interferometry has most frequently been used when imaging with resolution in excess of the diffraction limit is desired. A simple lens or mirror cannot image below .theta.=.lambda./D, where .theta. is the resolution angle in radians, .lambda. is the wavelength, and D is the entrance aperture of the telescope. For example, a visible light telescope one meter in diameter cannot resolve features finer than one tenth of an arc second. At radio wavelengths, single dish telescopes can be built with diameters as great as a hundred meters, but the large wavelength still limits the resolution to a relatively poor 15 arc seconds.
Through maintenance of phase information, interferometry allows one to sidestep the problem of gigantism, and build functional, affordable systems. Two telescopes linked together interferometrically can resolve features as if they were a single telescope with a diameter equal to the separation. The intercontinental baselines encountered in radio interferometry support resolution of milli-arcseconds, without constructing a telescope thousands of kilometers in diameter.
X-ray interferometry provides two fundamental advantages for exploration of the universe. First, since the wavelengths are much shorter, the baselines can be greatly reduced. The resolution achieved using the intercontinental baselines used by radio astronomers can be matched by an x-ray baseline of only 10 cm. Second, many astronomical x-ray sources are hugely bright, allowing imaging of tiny structures that in other wavelength bands would emit too little signal to be seen at interstellar distances.
Microscopy is not usually linked to interferometry because, in the visible band of the spectrum, radiation can be bent through large angles with high precision. The diffraction limit of a microscope sets in at resolutions of f.lambda., where f is the focal ratio of the lens and .lambda. is the wavelength of the light. Since f/1 lenses can be fabricated for visible light, there is no need to break the radiation into separate channels. This is not true at x-ray wavelengths, where each reflection is at most a few degrees. Consequently interferometry has a role to play in x-ray microscopy.
To make an x-ray interferometer highly sensitive and generally useful, it must have certain properties:
(a) The instrument must be efficient, including not only high gathering power, but broad band pass.
(b) It should be adjustable. It is impractical to build a new device every time a different resolution or wavelength is desired.
(c) The interference fringes should be well behaved and predictable, so that inversion algorithms will function reliably.
(d) The fringe patterns should be well matched to high efficiency electronic detectors that are affordable and well behaved.
(e) The components should be readily available, i.e., the instrument should not require some critical component that requires heroic technical efforts to fabricate or maintain.
3. Discussion of Prior Art
X-ray interferometers have been made from Laue crystals since 1965 (Bonse et al., App. Phys Lett, vol. 6, p. 155 (1965)). These crystal interferometers have enjoyed substantial success in a limited range of applications. Because the diffraction of x-rays by crystals is efficient over only a small range of wavelengths, these systems tend to be inefficient, and are typically used in conjunction with very bright sources, like synchrotrons. Thus they have been used for microscopy, but have no potential for astronomy, where the sources are intrinsically fainter.
For high efficiency interferometry, we need a device that will operate over a broad band with good response. U.S. Pat. No. 4,174,478 (Frank) suggested the x-ray equivalent of a Michelson interferometer to split the amplitude of a beam using a thin metal foil. However, this still has limited band pass, operating only near the wavelength at which the beam is split equally between transmission and reflection. Additionally, a working model has never been demonstrated.
The alternative is to use reflecting optics to combine different parts of the wavefront, as opposed to splitting the wavefront. The optics can be crystals, multilayers, or grazing incidence mirrors. Both crystals and multilayers (which are often referred to as synthetic crystals) have narrow spectral response which can be useful, but tends to make the system inefficient. Grazing incidence, however, can have excellent reflectivity across a broad band.
The classic interferometers for visible light were described in the nineteenth century during the early development of the field. Of these, two in particular use wavefront division and are adaptable for use at grazing incidence (Born et al., Principles of Optics, 6th ed., Pergamon, New York, (1993)). The first is the Lloyd's mirror interferometer, in which the direct beam is interfered against a part of the wavefront that has reflected at grazing incidence off a flat mirror. The other is the Fresnel bent mirror interferometer, in which two flat mirrors reflect a single wavefront. A small angular offset between the reflected beams leads to interference fringes where the beams subsequently overlap.
Kellstrom used both a Lloyd's mirror geometry and a Fresnel bent mirror to create x-ray fringes (Kellstrom, Nova Acta Soc. Sci. Upsala, vol. 8, p. 60 (1932)). The Lloyd's geometry, while creating fringes and demonstrating the principle, is extremely inefficient in collecting area, requiring the mirror to operate at a vanishingly small graze angle (i.e. below one arc minute). The Fresnel option can be used at larger angles and is the closest to satisfying the needs of a practical system.
4. Solution to the Problem
None of the prior art shows an interferometer for use at x-ray wavelengths that meets all of the conditions for practicality in terms of speed, cost, complexity, size, throughput, and tolerances for an x-ray system. It is the lack of a system that exhibits all these properties that has made x-ray interferometry a difficult, expensive, time consuming proposition for the microscopist, and an impossibility for the astronomer.
The present application presents and demonstrates a concept for an interferometer that exhibits these crucially important properties. It can be built with existing optical components. It exhibits high efficiency across a large band of the spectrum. It is adaptable to the requirements of astronomy or other scientific disciplines. The concept is practical because it solves the twin problems of: (a) how to mix the beams without losing signal; and (b) how to feed the signal into the beam mixer in a way that will support multiple spatial and spectral frequencies.
The present application shows there is a highly practical means for mixing the x-ray beams without a beam splitter. The principle is somewhat similar to that employed in the classic Fresnel bent mirror geometry, but the present configuration is particularly adaptable to grazing incidence.
The idea is to create two diffraction limited wavefronts, and steer them together at a small angle, as shown in FIGS. 1 and 2. These figures show a plane wavefront from infinity impinging on two flat mirrors. These flats steer the beams onto a second pair of flats that re-direct the beams into quasi-parallel convergence.
The beam converger must cross the beams at the detector at a very low angle. This leads to large fringe amplification. The wavelength of the fringes on the detector is given by .lambda.L/d, where d is the separation of the secondary mirrors at their centers and L is the distance from the centers of the mirrors to the detector where the beams cross. If L/d is large, the fringes can become macroscopic.